Plasma deposition refers to any of a wide variety of processes in which a plasma is used to assist in the deposition of thin films or coatings onto the surfaces of objects. For example, plasma deposition processes are widely used in the electronics industry to fabricate integrated circuits and other electronic devices, as well as to fabricate the magnetic tapes and disks used in audio, video, and computer applications. Plasma deposition processes may also be used to apply coatings to various objects to improve or change the properties of the objects. For example, plasma deposition processes may be used to apply wear resistant coatings to machine tools, while other types of coatings may be used to increase the corrosion resistance of other items, such as bearings, turbine blades, etc, thereby enhancing their performance. In still other applications, plasma deposition may be used to apply coatings to various types of surfaces in the optics and glass industries.
In most plasma deposition processes the plasma is created by subjecting a low-pressure process gas (e.g., argon) contained within a vacuum chamber to an electric field. The electric field, which is typically created between two electrodes, ionizes the process gas, creating the plasma. If direct current (DC) is used to produce the electric field, the negatively charged electrode is usually referred to as the cathode, whereas the positively charged electrode is referred to as the anode. Thus, in the case of a DC sputter deposition plasma process, the material to be deposited on the object or substrate is usually connected as the cathode, whereas some other element, typically the vacuum chamber itself, is connected as the anode. Ionized process gas atoms comprising the plasma are accelerated toward the negatively charged cathode which also includes a target containing the material to be deposited on the substrate. The process gas atoms ultimately impact the target material and dislodge or sputter atoms from the target, whereupon the sputtered atoms subsequently condense on various items in the chamber, including the substrate that is to be coated. The substrate is usually positioned with respect to the target so that a majority of the sputtered target atoms condense onto the surface of the substrate.
While sputter deposition processes of the type described above may be used to deposit a wide variety of metals and metal alloys onto various substrates, they may be used to deposit compound materials as well. Reactive sputter deposition is the name usually given to sputtering processes which involve the sputtering of the target in the presence of a reactive species (e.g., oxygen or nitrogen gas) in order to deposit a film comprising the sputtered target material and the reactive species. A wide variety of compounds, such as SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, and TiO, can be deposited by reactive sputter deposition processes.
The film deposited by such plasma deposition processes may be characterized by certain properties, such as adhesion; stress (i.e., compressive or tensile); stoichiometry; microstructure, including morphology, grain size, grain orientation, and epitaxy; hardness; abrasion resistance; density; as well as overall film thickness, just to name a few. Certain of these properties or characteristics may be of greater or lesser importance depending on the particular film and the type of application.
Unfortunately, while sputter deposition apparatus of the type described above are relatively easy to operate in a basic sense, the problem of operating sputter deposition apparatus to produce high quality films having the desired characteristics and properties on a repeatable basis is by no means trivial. Indeed, a significant portion of the current research efforts in sputter deposition technology are directed to developments and refinements of the sputter deposition apparatus and methods in order to improve the qualities of the deposited films.
While much remains to be learned about the mechanisms associated with film growth and the production of films having certain characteristics, certain mechanisms have been discovered that have predictable effects on film properties. For example, it has been found that the irradiation of the growing film with ions strongly affects film nucleation and growth, adhesion, film microstructure, and chemistry. Accordingly, many sputter deposition apparatus have been provided with separate ion sources (e.g., ion beams) to produce surface coatings having the desired properties. Unfortunately, however, the ion beam produced by a typical ion source is relatively small, typically a few millimeters in diameter, which limits the use of ion beam sputtering to applications involving like-sized substrates.
Partly in an effort to solve this problem, some operators have replaced the conventional narrow beam ion sources with ion thrusters of the type developed for space propulsion systems. Such ion thruster devices generally produce ion beams having relatively large cross sections, on the order of tens of millimeters, which allows larger substrates to be coated with the ion beam sputtering process. Unfortunately, problems still remain with regard to the maximum size of the substrate that may be effectively coated with such ion beam sputtering processes. Also, the provision to the sputtering system of an additional component (i.e., the ion beam source), increases the overall complexity of the system and may cause other problems.
Another method for controlling certain film properties that has been used with some degree of success is bias sputtering. In bias sputtering, the substrate is biased negatively with respect to the plasma potential. The negatively biased substrate attracts ions from the plasma, thereby providing low-energy ion bombardment of the growing film. With proper control of the low-energy ions, bias sputtering process may be used to achieve or control certain film properties. Unfortunately, however, bias sputtering is also not without its problems and limitations. For example, bias sputtering techniques cannot be used if the substrate is an electrical insulator. Also, the sputter deposition system must be provided with apparatus suitable for controlling the charge placed on the substrate to ensure that the ions bombard the substrate at the proper energy levels.
In summary, while the ion beam and bias sputtering processes described above are useful in certain applications, a need still exists for a film deposition system that will provide for the convenient control of certain specified film properties. Ideally, such a film deposition system should provide the operator with the desired degree of control of the specified film properties but without the need to resort to separate ion sources, with all their associated complexities and shortcomings. Additional advantages could be achieved if the desired film properties could be realized without the need to bias the substrate, as is required in bias sputtering processes.